overview

The mine currently operates on 10 metre benches with catch berms designed at varying intervals based on the geotechnical criteria. Haul road design is for a minimum width of 35m and a maximum gradient of 10%. The minimum planned mining width is generally 100m, but narrower widths are mined over short distances when unavoidable.

Ore material, which is not fed to the mill but is removed from the pit, is stockpiled. Waste material is broken down into several different types depending upon its competency and its metal leaching potential. Competent rock is placed in one of two dump areas. The Kogai dump is located to the south of the pit, with the North Anawe dump located to the east of the pit.

Monitoring of production is carried out by the engineering and geology group, augmented by a truck dispatch system.

Underground Mining Operations

The underground mine comprises four zones that are accessed using a general-purpose decline from surface. The mine is a highly mechanized bulk mining operation using four Atlas Copco development jumbos, six AD45 Elphinstone haul trucks, and a fleet of Elphinstone 2,900 and 1,700 LHDs. Production drilling is undertaken by two Tamrock Solo 1006 drills, one of which will be replaced by an Atlas Copco L6C drill in late 2007. Ancillary equipment consists of a fleet of service vehicles (including integrated tool carriers, scissor lift, dry-mix shotcrete machine, grader and a cablebolt drill).

Currently, open Stopes are filled with unconsolidated development waste, and cemented aggregate in strategic locations to create crown pillars.

PROCESSING

Run of mine ore is delivered by trucks to the dump pocket of a Fuller gyratory crusher. Primary crushed ore is conveyed to a coarse ore stockpile. A portion of the primary crusher product is diverted to a secondary crusher, with the secondary crushed product being delivered to the coarse ore stockpile. This additional crushing serves to increase the capacity of the SAG circuit.

The coarse ore recovered from the primary stockpile feeds two parallel 8.53m x 3.65m SAG mills. The SAG mills discharge over vibrating screens and the oversize product is directed to one of two Omnicone crushers where it is crushed prior to recycling to the SAG mill. The screen underflow is directed to the ball milling circuit (in closed circuit with cyclones). The cyclone underflow feeds three ball mills (mills 1 and 2 are 4.2 by 6.6 m long while ball mill 3 is 5.49 by 9.75 m long). A portion of the recirculating underflow is directed to Knelson concentrators for the gravity recovery of free gold. The gravity concentrate is collected in a storage tank for periodic transfer to an Acacia reactor located in the gold room. Cyclone overflow proceeds to flotation via 2x2km long gravity pipelines.

Flotation consists of rougher, cleaner, and scavenger banks producing a final concentrate of about 14% sulphur, and a final tail. The flotation concentrate is combined with the Acacia reactor tail and reground to 92% passing 38 µm. A concentrate thickener increases the pulp density of the reground product from 12% to 50% solids. Concentrate storage tanks provide approximately six days buffer between flotation and the oxidation section of the plant. As required, concentrate is pumped to a train of three carbonate reaction tanks, where the fresh feed is mixed with an acidic stream of recycled oxidized slurry to neutralize a majority of the carbonates in the concentrate. Carbonate destruction reduces the release of carbon dioxide in the autoclaves and thereby improves the utilization of oxygen. After carbonate destruction, the feed is directed to the four autoclaves. The autoclaves are 4m diameter, 27m long, steel pressure vessels that are lined with lead and acid-proof brick. The autoclaves are operated at 1,750kPa pressure and 197ºC. The reaction occurs autogenously once started, and water is added throughout the autoclave as necessary to control the reaction temperature. The process oxidizes approximately 98% of the sulphides. The oxidized slurry discharges from the autoclave under pressure, along with the vent gases, into a flash vessel that is equipped with a gas scrubber to control acidic emissions.

The autoclave discharge is treated in a wash circuit comprised of two stainless steel 35m diameter high-rate thickeners. The wash circuit operates counter-currently, using concentrate thickener overflow as the wash water. The washed and thickened slurry is fed to the cyanide leach circuit at 29% solids.

The leach circuit consists of seven agitated tanks. Milk of lime is added to the first tank to adjust the pH to 10.5. Sodium cyanide is added in the second leach tank to a level of about 150 g/tonne. At the conclusion of the leach step, a series of nine CIP (carbon-in-pulp) tanks recover the gold from the slurry. Each tank contains about six tonnes of carbon, with four tonnes of carbon forwarded each day. The Leach and CIP recovery is about 86-90%.

The elution circuit is comprised of two pressurized vessels that each holds approximately 10 tonnes of carbon. The precious metals are eluted from the carbon using 15 bed volumes of eluant at 140ºC and 400kPa. Barren carbon is regenerated in a rotary kiln and then acid washed in a 3% hydrochloric acid solution prior to being returned to the CIP circuit. Gold and silver are electrowon from the pregnant strip solution in three banks of cells. Each bank consists of three cells containing 18 stainless steel wool cathodes and 19 stainless steel mesh anodes. At regular intervals, the cathodes are removed and the gold “sludge” is washed off, pressure filtered, and retorted to remove any mercury. The mercury is condensed and collected as a by-product. The residue containing gold and silver is mixed with a flux of borax, soda ash, nitre, and silica, and smelted in an induction furnace to produce 500oz bars of doré bullion that average about 80% gold.

Acidic wash thickener overflow is combined with the CIP tailings for cyanide destruction, and further chemical treatment stabilizes any other potentially harmful constituents. This neutralization process has successfully kept metal levels under compliance levels.

Lime for neutralization purposes is produced from limestone quarried adjacent to the mine. The limestone is burned in two vertical kilns, trucked to the mill site, and stored in three lime silos. The burned lime is reacted with water to make milk of lime in a lime slaker, and is then circulated to the points of consumption within the plant.

Most of the water for the process plant is supplied by pipeline from the Waile Creek dam. The grinding plant preferentially takes water from the nearby Kogai Creek.

Electrical power is delivered to site via a 73km transmission line from the 62MW gas turbine Hides Power Station; this is supplemented by a 13MW diesel power station at the mine site.

PORGERA PROCESS PLANT FLOW SHEET

flowsheet

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